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Answer by Thomas Snerdley:
I don’t know exactly how Smaug got his weak spot. But I’m more than happy to speculate.
Tolkien faithfully continued the time-honored tradition of ancient European myth and legend in assigning his nearly invulnerable dragons with a single weak spot: “the underbelly.”
The first well-described mythological account of a monster that is essentially a dragon (“his mouth is fire, his breath is death”—sound familiar?) is that of Humbaba, which occurs in the Epic of Gilgamesh. The hero conquered the dragon by striking it while its guard was down (although the text isn’t definite about where exactly Gilgamesh struck, a sucker-punch to the belly isn’t out of the question), then chopping off its head.
In Tolkien’s beloved Old English poem Beowulf, a slave steals a golden cup from the hoard of a dragon at Earnaness, whereupon the dragon leaves its cave in a rage, burning everything in sight (sound familiar?). After the aged hero Beowulf broke his sword against the dragon’s invulnerably armored head, his kinsman Wiglaf stabbed his sword into the dragon’s belly, wounding it and enabling Beowulf to strike the death blow with his dagger.
The famous Catholic martyr St. George was heralded in legend for slaying a dragon, after shattering his spear against its armor, by driving his sword into the dragon’s underbelly (“under the wing where there were no scales”).
The Volsunga saga and the Nibelungenlied, two of Tolkien’s inspirations, see Odin advising the hero Sigurd to dig a pit and await the dragon Fafnir, which he slays by stabbing him in the belly with his enchanted sword Gram (sound familiar?).
Margaret the Virgin was said to have escaped being swallowed by satan in the form of a dragon because her cross burned its belly.
The terrible maiden-eating dragon of Kraków was slain by the Polish peasant Skuba, who tricked it into eating sulfur and drinking water, which caused its belly to explode.
This establishes a recurring pattern within Western European mythology and literature vis a vis dragons’ primary weak spot.
In Tolkien’s realm of Middle-earth, Morgoth was the creator of the original dragon, Glaurung, the progenitor of dragonkind in Middle-earth.
And guess where Glaurung’s weak spot was? The lord of the Naugrim (dwarves of Belegost) found it during the Nírnaeth Arnoediad:
… when in his rage Glaurung turned and struck down Azaghâl, Lord of Belegost, and crawled over him, with his last stroke Azaghâl drove a knife into his belly, and so wounded him that he fled the field …
Later when the human hero Túrin Turambar dealt Glaurung his death blow, guess where it came?
… he drew Gurthang, and with all the might of his arm, and of his hate, he thrust it into the soft belly of the Worm, even up to the hilts.
We are not told exactly how Eärendil the Mariner slew the mighty dragon Ancalagon the Black or how the hero Fram slew the dragon Scatha. But if I had to guess, I would say they got it in the relatively soft underbelly which they inherited from Glaurung, sire of the dragons of Middle-earth.
So it is quite curious that when our good friend Bilbo Baggins crept into Smaug the Golden’s lair, the dragon was actually sleeping with his only weak spot exposed:
Smaug lay, with wings folded like an immeasurable bat, turned partly on one side, so that the hobbit could see his underparts and his long pale belly crusted with gems and fragments of gold …
That was terrible operational security. But it would get even worse when Smaug vainly stood up to show off his entire underbelly to Bilbo.
But keep in mind that weak spot is a relative term. Glaurung was only slain by a very special sword forged by the dark elven smith Eöl of metal from a falling star (Gurthang, aka Anglachel). Smaug was only slain by a very special arrow forged by the dwarves of Erebor at the height of their magical and craft skill (the Black Arrow).
So the question remains: Why did Morgoth create Glaurung—and therefore the entire race of dragonkind—with a glaring weak spot in the underbelly? My unsubstantiated guess is that perhaps because Morgoth knew fear in his heart, he was too cautious to create any monster that was completely invulnerable. Good thing, too!
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